This invention relates to internal combustion engine carburettors comprising a starting device for improving the operation of the engine during the period after cranking.
It relates more particularly to carburettors having, located in the intake pipe, successively, in the direction opposite to that of air flow: operator operable main throttle means actuated by the user; a main fuel jetting system; and auxiliary throttle means urged in the closing direction by thermostatic means sensitive to the engine temperature when the engine temperature is less than a predetermined limit and urged in the opening direction by the air flow inspired by the carburettor and by a pneumatic element subjected to the pressure which prevails in the intake pipe, downstream of the main throttle means at least when said throttle means is in its minimum opening position, and constructed to open the auxiliary throttle means to a predetermined extent.
Usually such carburettors also comprise auxiliary means for preventing the main throttle means from closing to less than a minimum aperture which depends on the engine temperature, such as a "fast idle" cam (French Pat. No. 1,302,536).
In starting devices of this kind, a considerable depression prevails at the outlet of the main jetting system when the auxiliary throttle means (usually an eccentric throttle valve) is closed by the thermostatic means and the air fuel-mixture is quite rich while the motor is being driven by the starter. As soon as the engine becomes self-operative, the richness has to be rapidly decreased to prevent the engine flooding and stalling. For this purpose, the throttle valve has to be opened to a certain extent; this is done by the depression which builds up downstream of the main throttle means and which acts on the pneumatic element urging the starting valve in the opening direction. Similar carburettors are described in U.S. Pat. No. 2,348,544 (Jorgensen) and French Pat. No. 748,586. However, the pneumatic element disclosed in Jorgensen does not adjust a position, but changes the torque exerted by a thermostatic member. In the French patent, a pneumatic piston is provided, but its actuating linkage comprises a bimetallic spiral.
Most prior art starting devices of that type do not operate satisfactorily if the main throttle means is opened abruptly immediately after starting the engine when cold. Assuming the engine has just been cranked, while the main throttle means is in its minimum opening position, the pneumatic element partly opens the start valve. If, in order to accelerate the engine, the main throttle means is then opened beyond the minimum opening position determined by the "fast idle" cam and corresponding to the engine temperature, the flow rate of air through the intake pipe increases without a corresponding increase in the amount of fuel supplied by the main jetting system. Consequently, the air-fuel mixture supplied to the engine becomes leaner and engine operation becomes faulty.
It is an object of the invention to provide a carburettor comprising an improved starting device whose operation is more satisfactory than the existing devices. It is a more particular object to provide a carburettor which supplies to the engine a richer mixture immediately after the aforementioned opening of the main throttle valve after cranking, before the engine has reached its normal operating temperature.
According to an aspect of the invention, there is provided a carburettor comprising an additional pneumatic device which is actuated by the depression downstream of the main throttle means and is operatively associated with the auxiliary throttle means, whereby, when the main throttle means is abruptly opened while the engine is cold, the pneumatic device urges the auxiliary throttle means in the closing direction temporarily.
The pneumatic device typically comprises a main diaphragm connected to the auxiliary throttle means by a unidirectional or one-way mechanical connection whereby the pneumatic device can urge the auxiliary throttle means in the closing direction only, which diaphragm separates a chamber connected to a portion of the intake pipe which passes from downstream to upstream of the main throttle means when the throttle means is opened beyond its minimum opening position, from a second chamber communicating with the first chamber via a calibrated orifice. Advantageously, the second chamber is also connected to the first chamber by a non-return valve which opens when the depression in the first chamber is greater than the depression in the second chamber. The second chamber may be separated from atmosphere by an auxiliary diaphragm.
The main and auxiliary diaphragms form a movable unit which advantageously is biased by a return spring to an inoperative position where it does not exert any action on the auxiliary throttle means.
The invention will be better understood from the following description of a downdraught carburettor comprising a starting device and constituting an embodiment of the invention, given by way of non-limitative example. The description refers to the accompanying drawings.